An electric device, e.g., comprising a varistor, may catch fire under abnormal overvoltage conditions, which may be prevented by a thermal fuse. A thermal fuse device may have a hybrid design comprising an electric element that may be an electronic component, e.g., a varistor, and a single thermal fuse. The device is designed to integrate functions of the element and thermal fuse functions. Power supply is applied via a fuse electrode and an element electrode, the fuse and the element being connected in series. When long duration abnormal overvoltage is applied to the element, e.g., the varistor, the thermal fuse provided between the fuse electrode and the element will form an open circuit to disconnect the whole device from the power supply, thereby avoiding catching fire.
The device may include a monitor function. Such device has a monitor terminal. Before disconnection of the fuse electrode and the element electrode, the monitor terminal and the fuse electrode are short-circuited and after disconnection of the fuse electrode and the element electrode the monitor terminal and the fuse electrode are open circuit, which may be detected by or may provide a signal for an external device to identify whether the thermal fuse inside is open or not. Since a connection state between the fuse electrode and the monitor terminal usually changes from a closed to an open state, only one signal can be provided for an external device: change normally closed to open. Moreover, the monitor terminal which is usually connected with a warning alarm device in a low voltage circuit has to be connected with the fuse terminal in a high voltage circuit. Thus, a single thermal fuse device, e.g., comprising a varistor, may need a complex monitor circuit design in a customer's application. On the other hand double thermal fuse devices may be used. Such a device may comprise a varistor and further has a separation between low and high voltage circuits, both comprising independent standard thermal fuses. Since it is unsure whether both thermal fuses act under abnormal overvoltage conditions at the same time, wrong signals may be provided for an external warning alarm device.